The field of this invention relates to desiccant compressed gas dryers and techniques for regenerating them.
Many industrial processes require the supply of air for operation of control components. The Instrument Society of America (ISA) requires that the dew point of instrument air be kept below the coldest anticipated ambient air temperature so as to avoid condensation in the instrument air lines. Many installations set dew point limits far lower than those required by ISA for a variety of reasons. However, in many installations the level of dryness of the delivered compressed air is well below the actual system requirements.
To remove moisture from compressed air a plurality of towers are used. Each tower has a desiccant material and one tower is on line while another tower regenerates. Regeneration is periodically required because a tower becomes saturated with moisture and the dew point of the exiting air rises toward a preset set point. When this occurs, the spent tower in taken off line for regeneration and another tower that has concluded the regeneration cycle is put on line. The process is typically controlled automatically. The regeneration of a spent tower proceeds in three steps: heating, stripping, and cooling.
In the heating step, the exhaust gas directly from the compressor is directed into the tower, generally in the opposite direction as the air to be dried is normally fed in. The heat of compression from the compressor exhaust is used to drive the moisture off the desiccant. The exhaust air from the heating phase is run through a cooler and a separator to knock most of the water out before the gas is directed into the dryer that is on line for removal of the remaining moisture to the point that the desired dew point is achieved.
The heating cycle is normally done on a time basis or by sensing the gas outlet temperature from the tower being heated. When the controller senses that heating is complete, it shifts the valves so that the stripping cycle can begin. In the stripping cycle, some of the air dried from the tower that is on line is directed to the other tower, after the pressure is first reduced to nearly atmospheric. This stripping flow is cooled dried air, which helps to cool the desiccant bed and to remove any residual moisture from the tower after the heating cycle. The stripping stream is typically vented through a muffler. The stripping flow is typically 1-5% of the compressed gas flow. The purging of this much gas has a related energy cost of compression. Additionally, the compressor system may be running close to capacity and may not be able to meet system needs if 5% of the volume is vented for any significant time. While the stripping helps to reduce the dew point of the gas that will flow through the tower after regeneration is complete, it may do so well beyond the needs of many systems. Therein lies a potential to avoid energy waste if the regeneration performance is adapted to meet the system needs. This energy savings is the focus of the present invention. While past efforts to improve dryer performance have focused on the stripping step, they have addressed the situation where the compressor discharge temperature is low. With low compressor discharge temperatures, the regeneration of the dryer is not as effective and the desired dew point may not be achieved. To counter this problem, U.S. Pat. No. 6,375,722 provides a booster heater to heat only the stripping flow to compensate for the anemic heating cycle using low temperatures at the compressor discharge. The use of the stripping heater adds to energy cost. Again this system will produce air at dew points well below those required for most applications for instrument air. This reference does not address how to optimize the regeneration of a dryer so that over-drying of the air is avoided in order to save energy.
The last step in the regeneration sequence is the cooling cycle. Here, a slipstream of dried air from the tower that is on line is run into the tower being regenerated to cool it slowly. The cooling air rejoins the main airflow at the outlet of the drier that is on line to avoid the purging of any air from the system during the cooling phase. Cooling the desiccant allows the regenerated tower to go on line and produce very low dew points as desired by the system operator.
In the past, there have been unsuccessful attempts to eliminate the cycle with unacceptable fluctuations in the outlet dew point. The SP design of Henderson Engineering has this feature, which includes dew point excursions above the set point for as long as 8 minutes until the desiccant properly cools. Another design, offered as the MD dryer from Atlas Copco eliminates stripping by blending hot regeneration air with the cooler dry air from the on line tower to reduce the dew point spike. The problem with this design is that it is limited in how low a dew point can be produced and is more costly. This design lacks the flexibility that a dryer system that optimizes the stripping steps to meet system demands can achieve. Finally, heatless drying involves regeneration by a purge stream of dried air of approximately 15% of the gas compressed and dried at the time. It is not energy efficient due to the cost involved in compressing the volume that is purged to get the necessary drying.
Other U.S. Pat. Nos. that relate to the area of controlling and regenerating gas dryers are: 6,171,377; 6,221,130 and 5,632,802.
The present invention allows operation in an efficiency mode when air meeting ISA instrument air specification is called for. The system allows for high performance operation, when needed, to produce lower discharge dew points to meet the system requirements. The stripping cycle is reduced or eliminated depending on several parameters such as but not limited to: dew point required, the actual dew point, compressor discharge pressure, compressor discharge temperature, regeneration temperature, desiccant temperature, and ambient temperature. These and other aspects of the present invention will be readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the claims, which appear below.
A method to control the performance of desiccant dryers is disclosed that senses multiple variables and optimizes the regeneration cycle to deliver the gas at the desired dew point. The length of the stripping step is reduced or eliminated depending on the desired set point and the operating conditions of the compression system. The control system has the capability to adjust its mode of operation to meet new dew point requirements should the dew point set point be changed. The savings come from not purging as much or any gas during stripping should the system requirements be only to meet the ISA standards for instrument air, despite the system capability of delivering far dryer air.